1. Field of the Invention
The present invention relates to a micrometer for measuring a dimension of an object to be measured based on an axial displacement of a spindle.
2. Description of Related Art
A known micrometer for measuring a dimension of an object to be measured based on an axial displacement of a spindle is provided with a constant pressure mechanism easily operable by one hand (see Patent Literature 1: JP-A-2001-141402).
The micrometer includes: a body; a fixed sleeve that is fixed to the body; a spindle that is screwed to the fixed sleeve and is axially advanced and retracted; a thimble that is rotatably attached to an outer circumference of the fixed sleeve and has an outer end connected to an outer end of the spindle; an operation sleeve fitted over a range from an outer circumference of the thimble and the outer end of the spindle; and a constant pressure mechanism that is disposed between the outer end of the operation sleeve and the outer end of the spindle and allows a free rotation of the operation sleeve against the spindle when a predetermined or more amount of load is applied on the spindle.
For operating such a micrometer by one hand, while the body being held by one hand (with middle finger, fourth finger and fifth finger), the operation sleeve is operated by a thumb and an index finger. Then, the rotation of the operation sleeve is transmitted to the spindle through the constant pressure mechanism to advance the spindle. Since the constant pressure mechanism is operated to allow the operation sleeve to freely rotate when a tip end of the spindle is contacted with the object to apply a predetermined or more amount of load on the spindle, the object is measured with a measurement force at an approximately constant level.
However, in order to facilitate a one-hand operation, the above-described micrometer is arranged such that the operation sleeve is in a surface contact with the thimble to be rotated around the thimble.
On account of such an arrangement, when the operation sleeve is rotated, in addition to the measurement force generated in the constant pressure mechanism, a rotation force generated by frictional resistance between the operation sleeve and the thimble is transmitted to the spindle through the thimble. Particularly when the operation sleeve is rotated by one-hand operation, a force inclined relative to an axis of the operation sleeve is easily applied. Accordingly, the frictional resistance between the operation sleeve and the thimble is increased and the rotation force generated by the frictional resistance is easily transmitted to the spindle through the thimble. Consequently, the measurement force fluctuates, resulting in fluctuation of measurement values.